A known image forming apparatus includes a drum unit having arranged photosensitive drums and developing rollers disposed facing the photosensitive drums respectively, left and right main-body side plates supporting the drum unit positioned relative thereto, and a linear cam as a changing member which moves in a direction where the photosensitive drums are arranged to change positions of the developing rollers between a position where the developing rollers are separated from the photosensitive drums and a position where the developing rollers contact the photosensitive drums. In the image forming apparatus, the linear cam is held by a rail fixed to the main-body side plate such that the linear cam is movable in the direction where the photosensitive drums are arranged.